Method and apparatus for controlling moisture separator reheater

ABSTRACT

A system and a method are provided that may be used to control the temperature of steam being reheated by a moisture separator reheater (MSR). One embodiment provides a system including a steam turbine, a moisture separator reheater coupled to the steam turbine, and a controller programmed to control a temperature of steam leaving the moisture separator reheater based at least in part on sensor feedback. The controller is programmed to facilitate substantially smooth linear temperature ramping.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.13/934,151, filed on Jul. 2, 2013, which is a continuation of U.S.patent application Ser. No. 12/555,738, filed on Sep. 8, 2009, now U.S.Pat. No. 8,499,561, which are both incorporated by reference herein intheir entireties for all purposes.

BACKGROUND

The present invention relates to systems and methods for controlling amoisture separator reheater, for example, in power generation systemusing a steam turbine.

A variety of systems, such as power generation systems, utilize amoisture separator reheater (MSR) to dry and reheat a steam supply. Forexample, the MSR may dry and reheat steam in a steam turbine system thatdrives an electrical generator. In particular, the MSR may dry andreheat steam exhausted from a high pressure (HP) steam turbine, and thendeliver the dried, reheated steam into a low pressure (LP) steamturbine. The steam turbine system may acquire HP steam from a suitablesource, such as a boiler heated by a nuclear reactor or combustion of afuel-air mixture. The amount of heat transfer and moisture removal bythe MSR may affect overall performance of the steam turbine system, aswell as the power generation system. The MSR generally includes apneumatic controller responsive to a single control variable, i.e., theturbine pressure, indicative of turbine loading. Unfortunately, using asingle variable, open loop control system results in sub-optimalperformance and higher operating costs, because the turbine loadingrelates only indirectly to the MSR reheat process being controlled.

BRIEF DESCRIPTION

Certain embodiments commensurate in scope with the originally claimedinvention are summarized below. These embodiments are not intended tolimit the scope of the claimed invention, but rather these embodimentsare intended only to provide a brief summary of possible forms of theinvention. Indeed, the invention may encompass a variety of forms thatmay be similar to or different from the embodiments set forth below.

In a first embodiment, a system includes a controller capable ofcontrolling a temperature of an MSR through the use of a temperaturefeedback.

In a second embodiment, a system includes a closed-loop controllercapable of controlling the temperature of steam reheated by an MSRthrough the use of a temperature feedback.

In a third embodiment, a method includes sensing the temperature of thesteam being reheated by an MSR and controlling the reheating of thesteam by using a closed-loop control based at least in part on thesensed temperature.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features, aspects, and advantages of the presentinvention will become better understood when the following detaileddescription is read with reference to the accompanying drawings in whichlike characters represent like parts throughout the drawings, wherein:

FIG. 1 is a diagram of an embodiment of a nuclear power plant;

FIG. 2 is a diagram of an embodiment of a MSR control and regulationsystem to control first and second reheater stages of a plurality ofMSRs;

FIG. 3 is a diagram of an embodiment of a plurality of PID controllerseach configured to control a valve of a second stage reheater of a MSR;

FIG. 4 is a flowchart of an embodiment of control logic configured todetermine whether to issue a temperature ramping hold command;

FIG. 5 is a flowchart of an embodiment of control logic that may be usedin conjunction with FIG. 6 to operate a second stage reheater of a MSR;and

FIG. 6 is a graph of an embodiment of a temperature loading model thatmay be used in conjunction with FIG. 5 to operate a second stagereheater of a MSR.

DETAILED DESCRIPTION

One or more specific embodiments of the present invention will bedescribed below. In an effort to provide a concise description of theseembodiments, all features of an actual implementation may not bedescribed in the specification. It should be appreciated that in thedevelopment of any such actual implementation, as in any engineering ordesign project, numerous implementation-specific decisions must be madeto achieve the developers' specific goals, such as compliance withsystem-related and business-related constraints, which may vary from oneimplementation to another. Moreover, it should be appreciated that sucha development effort might be complex and time consuming, but wouldnevertheless be a routine undertaking of design, fabrication, andmanufacture for those of ordinary skill having the benefit of thisdisclosure.

When introducing elements of various embodiments of the presentinvention, the articles “a,” “an,” “the,” and “said” are intended tomean that there are one or more of the elements. The terms “comprising,”“including,” and “having” are intended to be inclusive and mean thatthere may be additional elements other than the listed elements.

The disclosed embodiments include systems and methods for controlling amoisture separator reheater (MSR) based on temperature feedback and/ormultiple feedback parameters. MSRs may be used in industrial processes,particularly in nuclear power plants, that operate wet steam turbines.In such plants, saturated or near saturated steam produced by a boilermay be used to power a high pressure (HP) turbine and may also be usedby a plurality of MSRs to reheat an exhaust steam from the HP turbine.More specifically, an MSR may contain two reheater stages for use inreheating the exhaust steam from the HP turbine. The reheated exhauststeam may then be routed into a low pressure (LP) turbine and used asthe LP steam that drives the LP turbine. The MSR's first stage reheatermay be supplied with an extraction (bleed) steam from the HP turbine,and the MSR's second stage reheater may be supplied a main steam, i.e.the steam produced by the boiler. The two MSR reheater stages may thenreheat the exhaust steam leaving the HP turbine and feed the reheatedexhaust steam into a LP turbine as LP superheated steam. The MSR mayincrease the overall energy efficiency of the plant by removing excessmoisture from the HP exhaust and by improving the thermalcharacteristics of the reheated LP steam (i.e., a LP steam with optimaltemperature and pressure for use in rotating the LP turbine).

Unfortunately, a control system based solely on a turbine pressureindicative of turbine load does not provide an accurate control of thereheat temperature of the MSR. In particular, a non-linear relationshipexists between a reheat temperature of the MSR and a valve position ofsteam being supplied to the MSR. As a result, a linear increase inpressure causes inaccurate control of the MSR due to the non-linearrelationship. Furthermore, a control system based solely on turbinepressure ignores a variety of variables that may impact operation of theMSR and the entire system.

As discussed in further detail below, the disclosed embodiments includea controller, control logic, and various control techniques to improveoperation of the MSR based on multiple feedback parameters, such astemperature, pressure, vibration, fluid flow rate, clearance, valveposition, or any combination thereof. Furthermore, the disclosedembodiments may include a digital control system coupled to multiplesensors and actuators (e.g., valves), which are distributed throughoutthe system at multiple locations. The sensors may be configured tosupply electronic feedback signals, while the actuators may beresponsive to electronic control signals. Alternatively, the sensors andactuators may operate with analog signals. The actuators (e.g., valves)may control the flow of steam to the steam turbine stages, the MSRstages, and so forth. For example, the digital control system may be adistributed, closed-loop control system configured to control the secondstage reheater of one or more MSRs.

FIG. 1 is a diagram of an embodiment of a power plant 10 having anuclear steam boiler 12 as a main steam source 14. Alternatively, themain steam 14 may be produced by, for example, a fossil fuel-poweredboiler 12. The main steam 14 is supplied to a main turbine 16, whichuses the main steam 14 to rotate a turbine shaft 18. An electricalgenerator 20 converts the mechanical rotation of the turbine shaft 18into electricity, which may then be used to power an electrical load 22,e.g., a power grid.

In certain embodiments, the main turbine 16 includes one or more highpressure (HP) turbines 24 and one or more low pressure (LP) turbines 26.Other embodiments may also include one or more intermediate pressure(IP) turbines. In the illustrated embodiment of FIG. 1, a single HPturbine and a single LP turbine is used by the power plant 10. The mainsteam 14 produced by the boiler 12 may be directed to the HP turbine 24through a control and stop valve 28. The main steam 14 flows through theHP turbine 24, thereby driving a plurality of turbine blades to rotatethe shaft 18. The steam expands and increases in moisture contentthrough the HP turbine 24, and exits as an exhaust steam 30. The exhauststeam 30 from the HP turbine's exhaust may still contain usable energy.However, the exhaust steam 30 may also contain excessive moisture, insome cases upwards of 25% water. An MSR 32 may remove the moisture fromthe exhaust steam 30 by using moisture separation embodiments 31 and mayreheat the exhaust steam 30 so that it may be more efficiently used bythe low pressure turbine 26.

The reheating system of the MSR 32 may include at least two stages ofheat exchange. Extraction steam 34 may be directed from the HP turbine24 through a reheat steam control and stop valve 36 and used to feed afirst stage reheater 38 (e.g., heat exchanger) of the MSR 32. Forexample, the extraction steam 34 may pass through (e.g., internal to)tubes of a fin and tube heat exchanger 38, while the exhaust steam 30flows around (e.g., external to) the heat exchanger 38, therebytransferring heat from the extraction steam 34 to the exhaust steam 30.Main steam 14 may be directed from the boiler 12 through a reheat steamcontrol and stop valve 40 and used to supply a second stage reheater 42(e.g., heat exchanger) of the MSR 32. For example, the main steam 14 maypass through (e.g., internal to) tubes of a fin and tube heat exchanger42, while the exhaust steam 30 flows around (e.g., external to) the heatexchanger 42, thereby transferring heat from the main steam 14 to theexhaust steam 30. Superheated steam 44 heated through the use of thefirst stage reheater 38 and the second stage reheater 42 may then exitthe MSR's 32 outlets and be directed into the inlets of the LP turbine26 through a turbine reheat steam control and stop valve 46. The LPturbine 26 may then convert the thermal energy in the reheated exhauststeam 44 into mechanical energy, which may be used to rotate the turbineshaft 18. A LP turbine exhaust steam 48 may then be directed into acondenser 50 in order to recover water for use in other plant components(e.g., as feedwater). Accordingly, the MSR 32 reheating system may beused to improve the performance and reliability of the LP turbine 26 byproviding thermally efficient steam (i.e., steam that enters the LPturbine at a temperature and pressure optimal for conversion intorotational energy).

As further illustrated in FIG. 1, the system 10 includes an exemplaryembodiment of a turbine-generator control system 52. In certainembodiments, the control system 52 may be a digital control system orcomputer-implemented control system using electronic sensor feedback andelectronic actuators (e.g., valves). The turbine generator controlsystem 52 may manage the various main turbine 16 systems, the generator20 systems, and related components (e.g., the MSR 32, valves 28, 36, 40,and 46) so as to operate the systems and components in a safe andefficient manner. The turbine-generator control system 52 may besubdivided into multiple sub-control systems, e.g., a supervisorycontrol system 54 and a rotating equipment protection system 56. Thesupervisory control system 54 may control the operation of the mainturbine 16 systems and related components along with the generator 20 sothat the generator 20 may optimally generate electricity at a frequency(e.g., 60 Hz) adequate to power the electrical load 22. The rotatingequipment protection system 56 may oversee the various plant systemsrelated to rotational equipment (e.g., the turbine shaft 18) and ensurethat the systems stay within safe operational parameters.

The supervisory control system 54 may be further subdivided intomultiple sub-control systems, e.g., a speed/load control and regulationsystem 58 and an MSR control and regulation system 60. The speed/loadcontrol and regulation system 58 may control the speed and the loadingof the main turbine 16 to control thermal gradients, clearance,stresses, and performance of the turbine 16, as well as to generate thedesired electricity based on demands. For example, the regulation system58 may increase or decrease the speed of the turbine 16 based onelectrical demand, adjust clearance based on steady state or transientconditions, and so forth. In other words, the speed/load control andregulation system 58 may provide a suitable match between the speed andloading of the turbines 24 and 26, so that the turbines 24 and 26 staywithin their operational parameters while minimizing fuel consumptionand providing sufficient electricity to meet the demands. As discussedin further detail below, the MSR control and regulation system 60 maymanage the reheating stages 38 and 42 of the MSR 32 using multiplefeedback signals, e.g., temperature, pressure, vibration, clearance,moisture content, valve position, shaft speed, load, fluid flow rate, orany combination thereof.

FIG. 2 is a diagram of an embodiment of the MSR control and regulationsystem 60 that may be used to control the reheating stages of aplurality of MSRs 32. As discussed in detail below, the system 60 is adigital closed-loop control system that adjusts actuators (e.g., valves)based on a variety of sensors feedback, including temperature feedback.For example, the system 60 may be responsive to electronic sensorfeedback signals indicative of temperature, pressure, vibration,clearance, rotational speed, fluid flow rate, valve position, load, andso forth. There are several modes of turbine operation (e.g., loading,unloading, trip, auto shutdown, and parasitic loss control) that usespecific control of the steam fed into a MSR 32. Accordingly, the MSRcontrol and regulation system 60 may use certain controller embodimentsto properly and efficiently control the steam fed into the two reheaterstages of a MSR 32 as described in more detail with respect to FIGS. 3,4, 5 and 6 below. In the embodiment of FIG. 2, the MSR control andregulation system 60 is used in the power plant 10, which includes onedouble-flow HP turbine 24, three double-flow LP turbines 26, and fourMSRs 32. The MSR control and regulation system 60 may include threesubsystems, a coordinated clearance management system 62, a thermal ratecontrol system 64, and a temperature control system 66.

The coordinated clearance management system 62 may be used to processvibration data sensed by a set of vibration sensors Vx 68 (e.g., V1, V2,V3, V4, and V5) and temperature data sensed by a set of temperaturesensors Tx 70 (e.g., T1, T2, T3, T4, T5, and T6) in order to activelyadjust clearance between the rotating and stationary components of theLP turbines 26, in order to, for example, prevent clearance rubs. Thethermal rate control system 64 may be used to process valve positiondata sensed by a set of valve position sensors Zx 72 (e.g., Z1, Z2, Z3,Z4, Z5, and Z6) in order to actively adjust the thermal rate and theloading of the LP turbines 26, in order to, for example, follow thethermal requirements of various modes of MSR operation (e.g., loading,unloading, trip, auto shutdown, parasitic loss control). The temperaturecontrol system 66 may be used by both the coordinated clearancemanagement system 62 and the thermal rate control system 64 to controlthe temperature of the second stage of an MSR 32 by actively adjustingthe position of a reheating steam low load valve (RSLLV) 74 and areheating steam high load valve (RSHLV) 76 of each of the four MSRs 32.The temperature control system 66 may also process data sensed by theset of temperature sensors Tx 70 (e.g., T1, T2, T3, T4, T5, and T6) andpressure/loading sensors Px 69 (e.g., P1, P2, and P3) and use certaincontroller embodiments described in more detail with respect to FIG.3below to control the reheating of the steam that will be directed intothe LP turbine 26.

It is to be understood that sensors Vx 68, Px 69, Tx 70 and Zx 72 may befound throughout the power supply plant 10 and are not limited to thoseshown in FIG. 2. Other sensors not shown include speed sensors,clearance sensors, and flow sensors. Many of the sensors may be reusedfrom other control applications. For example, the speed/load control andregulation system 58 of FIG. 1 may include a set of rotary speed sensorsthat may be affixed to various points on the turbine shaft to measurethe turbine speed. The rotating equipment protection system 56 mayinclude clearance sensors affixed to the turbine shroud and used tomeasure, for example, rubs between the shaft and the shroud. Flowsensors may be affixed to supply lines and used to measure the volume ofsteam flowing through the lines. Temperature sensors may includethermocouples, thermistors, resistance temperature detectors (RTDs),bimetallic, infrared, and others. Vibration sensors may includeaccelerometer sensors, displacement sensors, velocity sensors, or acombination thereof. Pressure sensors may include strain gauges,diaphragm sensors, load cells, differential pressure transducers, andothers. Valve position sensors may include inductive position sensors,rotary encoder sensors, proximity sensors, limit switches, and others.

As mentioned above with respect to FIG. 1, each MSR 32 includes a firststage reheater 38 and a second stage reheater 42 that may be controlledto reheat the exhaust steam 30 from the HP turbine 24. Both reheaterstages 38 and 42 may be interlocked such that the second stage 42 maynot operate unless the first stage 38 of the MSR is also in service. Thefirst stage reheater 38 may be controlled by controlling the extractionsteam 34. Extraction steam 34 coming from the HP turbine 24 may bedirected into the first stage reheater 38 through the reheating steamcontrol valve (RSCV) 36. In certain embodiments, the RSCV 36 is anopen/close check valve. The flow of extraction steam 34 through the RSCV36 varies according to the loading of the extraction steam 34 source,i.e., the HP turbine 24. As appreciated, a reduction in flow of theextraction steam 34 results in a reduction of heat transfer to theexhaust steam 30, while an increase in flow of the extraction steam 34results in an increase in heat transfer to the exhaust steam 30. Incertain embodiments, the RSCV 36 valve may be left fully open as soon asthe turbine shaft 18 reaches a rated speed. Extraction steam 34 enteringthe first stage reheater 38 from the HP turbine 24 may then varyproportional to the loading of the HP turbine 24.

The second stage reheater 42 may be controlled by controlling the mainsteam 14. Main steam 14 coming from the boiler 12 may be directed intothe second stage reheater 42 through two valves, a RSLLV 74 and a RSHLV76. The RSHLV 76 is connected in parallel with the RSLLV 74 and may beopened at higher loads in order to reduce parasitic pressure drop acrossthe RSLLV 74. The RSLLV 74 may be opened and closed (i.e., modulated) bya closed-loop controller as described in more detail with respect toFIG. 3 below. The operation of the RSCV 36, RSLLV 74 and RSHLV 76controls the flow of extraction steam 34 entering the first stagereheater 38 and the flow of main steam entering the second stagereheater 42, thereby controlling the heat transfer to the HP exhauststeam 30. The thermally enhanced steam 44 may then be directeddownstream into the LP turbine 26 through a combined intermediate valve(CIV) 78. The LP steam may then be converted into mechanical energy bythe LP turbine 26 and used to rotate the turbine shaft 18. The rotatingshaft 18 may then be used by the generator 20 to generate electricityfor distribution to an electrical load 22.

FIG. 3 is a diagram of an embodiment of a set of proportional integralderivative (PID) controllers 80 that may be used to control the RSSLV74. More specifically, the MSR control and regulation system 60 mayinclude a set of the PID controllers 80 such as those depicted in FIG. 3and use the PID controllers 80 to modulate (i.e., incrementally open andclose) a RSSLV 74. Each PID controller 80 controls a separate MSR 32.That is, the four PID controllers 80 depicted in FIG. 3 are capable ofoperating the four RSSLV 74 depicted in FIG. 2. Each one of the PIDcontrollers 80 controls the valve position of a RSSLV 74 that allowsmain steam 14 to enter the second stage reheater 42 of the MSR 32. Forexample, the PID controller 80 driving the RSSLV-1 74 controls theexhaust steam entering the second stage reheater 42 of the MSR-1connected to the RSSLV-1 74 of FIG. 2. Similarly, the PID controller 80driving the RSSLV-2 74 controls the exhaust steam 30 entering the secondstage reheater 42 of the MSR-2 connected to the RSSLV-2 74. The PIDcontroller 80 driving the RSSLV-3 74 controls the exhaust steam 30entering the second stage reheater 42 of the MSR-3 connected to theRSSLV-3 74 and the PID controller 80 driving the RSSLV-4 74 controls theexhaust steam 30 entering the second stage reheater 42 of the MSR-4connected to the RSSLV-2 74.

The PID controller 80 embodiments depicted in FIG. 3 may use an outlettemperature sensed by the temperature sensor Tx 70 as the process value(PV) 82 input. A temperature reference setpoint (SP) 84 may becalculated using embodiments described in more detail with respect toFIGS. 5 and 6. A PID control block 86 may use proportional integralderivative techniques to define a closed-loop temperature control basedon the reheat temperature feedback given by the temperature sensor Tx 70and the temperature reference setpoint 84. In one embodiment, the outputo 85 of the PID control block 86 may be calculated using the equationbelow:

o=P[(PV−SP)+I∫(PV−SP)+D(dPV/dt)]

In the equation above, P is the proportional gain, I is the integralgain, and D is the derivative gain that may be used by the PID controlblock 86 when calculating the next output o 85 based on the currentinput variables PV 82 and SP 84. The gains P, I, and D may be derivedfor specific installations of the second stage reheater 42 by usingsuitable PID tuning techniques, for example, the Ziegler-Nichols method,the Cohen-Coon method, manual tuning, and/or by using software toolsdeveloped for PID tuning. An operator may tune the PID controller 80 fora specific second stage reheater 42 installation in order to arrive atspecific gains P, I, and D.

The PID control block 86 may be constantly updating, that is, receivingthe input setpoint 84 and feedback from the temperature sensor Tx 70(i.e., as process value 82) and may be using the equation above toderive a new output o 85. The output o 85 may then be used to modify thevalve position of the RSSLV 74. This process of receiving new inputs(e.g., process value 82, setpoint 84) and determining an output o 85repeats cyclically very quickly, in some embodiments, every fewcomputing cycles of a microprocessor. The output o 85 may then be usedto adjust the RSSLV 74 position accordingly. By constantly adjusting,i.e., modulating, the position of the RSSLV 74, the PID controller 80may control the precise volume of steam entering the second stagereheater 42 of the MSR 32 so that the MSR 32 may optimally reheat a HPexhaust steam 30.

In the illustrated embodiment of FIG. 3, the controller 80 also includesa ramp control block 88 that may be used to derive the new setpoint 84.The ramp control block 88 may derive the new setpoint 84 by ramping upor down a given amount from the previous setpoint 84. This ramping up ordown from the previous setpoint 84 may be used to prevent excessivechanges in the setpoint 84 that may lead to thermal distortions andinefficiencies. Accordingly, the new setpoint 84 may only be allowed tovary, for example, by approximately 10, 20, 30, 40, or 50° F. from theprevious setpoint 84. In one embodiment, the ramp control block 88 mayhave three input variables, a RSLLV temperature reference 90, a maximumallowable ramp rate 92, and a temperature ramping hold command 94. Inone embodiment, the RSSLV temperature reference 90 may be derived byusing a function F (L, T, V, Z) as described in more detail with respectto FIGS. 5 and 6 below. In another embodiment, the RSSLV temperaturereference 90 may be derived by using a RSSLV temperature reference axis(ordinate axis) of a temperature loading model as described in moredetail with respect to FIGS. 5 and 6 below. The maximum allowable ramprate 92 may be a constant that determines an upper value ramping ratefor safely operating the MSR 32, derived, for example, by respecting thethermal limitations of specific MSR embodiments. The temperature rampinghold command 94 is used to hold, i.e. maintain, the same value for thetime interval, the temperature setpoint 84. As discussed further below,FIG. 4 is a flowchart of an embodiment of a process that may be used toderive when to issue the temperature ramping hold command 94.

Continuing with FIG. 3, the ramp control block 88 may first determinewhether or not the temperature ramping hold command 94 has been set. Ifa temperature ramping hold command 94 has been set, then the rampcontrol block 88 maintains the temperature setpoint 84 at its currentvalue. The temperature setpoint 84 may stay at the same hold value foras long as the temperature ramping hold command 94 is set. If notemperature ramping hold command 94 has been set, then the ramp controlblock 88 may calculate a new RSLLV temperature reference value 90 byusing the embodiments of FIGS. 5 and 6 described below, e.g., as afunction F (L, T, V, Z) or a RSSLV temperature reference axis of atemperature loading model.

When ramping up, i.e., when the newly calculated RSLLV temperaturereference value 90 is greater than the existing setpoint 84, the rampcontrol block 88 may check whether the newly calculated RSLLVtemperature reference value 90 is greater than the existing temperaturesetpoint 84 by a maximum allowable ramp rate 92 amount (i.e., if thenewly calculated RSLLV temperature reference value 90<setpoint value84+maximum ramp rate value 92). If the newly calculated RSLLVtemperature reference value 90 is not greater than the existingtemperature setpoint 84 by a maximum allowable ramp rate 92 amount, thenthe newly calculated RSLLV temperature reference 90 value may be used asthe new temperature setpoint 84. If the newly calculated RSLLVtemperature reference 90 value is greater than the existing temperaturesetpoint 84 by a maximum allowable ramp rate 92 amount (i.e., if thenewly calculated RSLLV temperature reference value 90>setpoint value84+maximum ramp rate value 92), then the new setpoint value 84 may becalculated by adding the old setpoint value 84 to the maximum allowableramp rate 92 (i.e., old setpoint value 84+maximum allowable ramp rate92).

Similar logic may be used by the ramp control block 88 to ramp down froma previous setpoint value 84, i.e., when the newly calculated RSLLVtemperature reference value 90 is less than the previous setpoint value84. If the newly calculated RSLLV temperature reference value 90 is notless than the existing temperature setpoint 84 by a maximum allowableramp rate 92 amount (i.e., if the newly calculated RSLLV temperaturereference value 90>setpoint value 84−maximum ramp rate value 92), thenthe newly calculated RSLLV temperature reference 90 value may be used asthe new temperature setpoint 84. If the newly calculated RSLLVtemperature reference 90 value is less than the existing temperaturesetpoint 84 by a maximum allowable ramp rate 92 amount (i.e., if thenewly calculated RSLLV temperature reference value 90 <setpoint value84−maximum ramp rate value 92), then the new setpoint value 84 may becalculated by subtracting the old setpoint value 84 to the maximumallowable ramp rate 92 (i.e., old setpoint value 84−maximum allowableramp rate 92). Accordingly, the ramp control block may allow forincremental control (i.e., ramping up or down) over the setpoint 84temperature, insuring that the MSR's stage reheater temperatures do notvary excessively and stay within safe operating limits.

The controller 80 also includes control logic that may be used for CIV78 testing for the MSR 32, i.e., a valve test mode 96 of the CIV 78. Incertain embodiments, the controller 80 periodically tests the CIVs 78 toensure that they are working within their design parameters, that is,that the CIVs 78 can completely open and close. During testing, the CIVs78 may be set to the valve test mode 96 and then intentionally moved toa fully closed position. The temperature setpoint 84 may be held duringtesting of CIVs 78 so that the valve test can occur without anyunnecessary temperature transients in LP turbine 26 sections during orjust after the valve test. Thus, a temperature ramping hold command 94may be issued when the CIV 78 is in the valve test mode 96.

While the embodiment of FIG. 3 depicts the PID controller 80, it is tobe understood that other embodiments may be used. For example, aprogrammable logic controller (PLC), a computer, an embedded system, andothers may used in lieu of the PID controller 80 of FIG. 3. Acombination of different controller types may also be used, that is,some of the control logic of the PID controllers 80 depicted in FIG. 3may be embodied not only in PIDs 80, but also in PLCs, computers,embedded systems, and/or a combination thereof. It is also to beunderstood that controller 80 values such as the maximum allowable ramprate 92 may be easily changed by simply entering a new value into thecontroller. The controller 80 may include a graphical user interface(GUI) to enter values, to reprogram existing functionality, to issuecommands (e.g., temperature ramping hold command 94), and others.

FIG. 4 is a diagram of an embodiment of a process to derive thetemperature ramping hold command 94 mentioned above. A process step 97may first find the difference between two temperatures sensed bytemperature sensors 70 positioned on opposite sides of a double-flow LPturbine 26 (shown in FIG. 2) and then find the absolute value of thedifference between the two temperatures at step 98. For example, if thetemperature sensors T1 and T6 are being used, the step 98 will calculate|T1-T6|. T1 and T6 may have been chosen because the two temperaturesensors T1 and T6 are sensing temperatures from opposite sides of thesame double-flow LP turbine LPC 26 (shown in FIG. 2). The reduction ofside-to-side temperature differences is important in reducing thepossibility of unwanted side-to-side shifting of the LP turbine LPC 26.In particular, side-to-side temperature differences can result indifferent thermal expansion from one side to another (i.e., side-to-sideshifting), thereby causing differences in the clearances from one sideto another. In turn, the variation in clearances from one side toanother may result in unwanted effects such as vibration. In order toprevent such side-to-side shifting, the absolute value of theside-to-side temperature difference between two opposing temperaturesensors 70, for example sensors T1 and T6, may be compared to a maximumallowable difference temperature 100 at a step 102. If the side-to-sidetemperature difference found at step 98 is larger than a maximumallowable difference temperature 100, or it is found that a CIV 78 is inthe valve test mode 96, then a temperature ramping hold command 94 maybe issued at step 104. Other side-to-side temperature differencescalculated include the temperatures sensed by the sensor pair T2 and T5,and the sensor pair T3 and T4.

Should any of the side-to-side temperature differences calculated byusing the sensor pairs, T1-T6, T2-T5, or T3-T4 result in a side-to-sidetemperature difference exceeding the maximum allowable differencetemperature 100 or it is found that a CIV 78 is in the valve test mode96, then the temperature hold command 94 resulting from step 104 may becommunicated to the PID controllers 80. For example, if the side-to-sidetemperature difference sensed by the temperature sensor pair T3-T4results in a temperature hold command 94, then the hold command 94 maybe communicated to the PID controllers 80 that control valves RSSLV-2and RSSLV-3 (shown in FIG. 3). Similarly, if any of the CIVs 78 is inthe CIV valve test mode 96 then the temperature hold command 94resulting from step 104 may be communicated to the appropriate PIDcontrollers 80.

The PID controllers 80 may collaboratively act to reduce the possibilityof temperature gradients. For example, the PID controllers may use thesame temperature sensors and take appropriate control actions to reducetemperature differences. By minimizing the temperature differences, thecontrollers are able to reduce the possibility of temperature gradientscausing variations in clearances. Less clearance variations will lead toless shifting and consequently, to the reduction of vibrations. Thus,the logic depicted in FIG. 4 prevents unwanted side-to-side temperaturedifferences as well as allow for proper valve control during CIV 78testing.

FIG. 5 is a flow chart of an exemplary embodiment of control logic thatmay be used in conjunction with a temperature loading model 106 of FIG.6 to control the second stage reheater 42 of the MSR 32 during variousoperating modes. As discussed above, the steam fed into the MSR 32 isincreased or decreased in a controlled manner to improve performance andreduce the possibility of shock to the system. For example, there areseveral operating modes of the second stage reheater 42 that may utilizespecific control logic for controlling the steam fed into the secondstage reheater 42. Accordingly, a second stage reheater 42 operatingmode control block 108 may be used to execute control logic for fiveoperating modes of the second stage reheater 42. The five operatingmodes each have a control block as follows. A loading control block 110may be used to control operations during loading, e.g., when there is anincrease in main steam 14 being directed into the turbine system 16. Aparasitic loss control block 112 may be used to control the loss ofpressure when operating at high loads. An unloading control block 114may be used to control operations during unloading, e.g., when there isa decrease in the main steam 14 being directed into the turbine system16. A trip control block 116 may be used to control operations whenthere is a trip condition for a very rapid shutdown of the MSR 32. Anauto shutdown control block 118 may be used to shut down the MSR 32 in acontrolled manner, but not as rapid as the trip control block 116.

Returning to the loading control block 110, the control block 110 mayuse the loading model 106 of FIG. 6 to arrive at a RSSLV temperaturereference value 90 that may then be used by closed-loop controllerembodiments as described above in FIG. 3 to arrive at a temperaturesetpoint 84. In a decision 120, the current turbine load percentage iscompared to load point U (abscissa) of the temperature loading model 106of FIG. 6. If the current turbine load percentage is greater than loadpoint U, then the control logic moves to decision 122. If the currentturbine load is not greater than load point U, then no action may needto be taken. Traditionally, load point U has been chosen to beapproximately 15% of the turbine loading. Accordingly, the MSR secondstage reheater 42 has traditionally not been turned on until loadingreaches 15%. However, by using the disclosed closed-loop controlembodiments, the load point U need not be at 15% of loading anymore.Indeed, the load point U may be anywhere from approximately 15% to 50%of loading depending on factors such as turn-downs and outage needs.Further, the load point U may be updated and easily changed through theuse of, for example, a GUI.

At decision block 122, the process evaluates whether the currentoperational state of the MSR is along a loading line 124 of MSRtemperature versus load. For example, a point (a, b) represents thecurrent operational state, wherein a represents the current turbine loadpercentage and b represents the current sensed temperature. If thecurrent sensed temperature (e.g., second stage reheater 42 outputtemperature) is not on the loading line 124 of FIG. 6, then at step 126the current RSLLV temperature reference value 90 is held, i.e., kept atthe same value, and the control logic repeats decision 122. The loadingline 124 of FIG. 6 represents an exemplary relationship between thecurrent turbine loading percentage and the ideal temperature at theRSSLV. That is, the RSSLV temperature that should be used to reheatsteam when the turbine loading is at a given %. Ideally, the point (a,b) tracks the loading line 124 during loading operations. Thus, if thepoint (a, b) is outside the loading line, then the RSSLV temperaturereference 90 may be held while loading in order to allow the point (a,b) to return to the loading line 124. The point (a, b) need not beperfectly on the loading line 124 and in some embodiments the point (a,b) may be considered to be on the loading line 124 if the point (a, b)is within a certain tolerance of the loading line 124. For example, thetolerance may be plus or minus less than approximately 1, 2, 3, 4, or 5percent of the temperature on the loading line, e.g., plus or minus 5,10, 15, or 20 degrees Fahrenheit. In certain embodiments, the loadingline 124 depicted in FIG. 6 may have a different slope or shapedepending on factors including the type, manufacture, and materialproperties of the turbine and MSR components as well as the type ofcontrol desired (e.g., linear versus non-linear). Indeed, in someembodiments of the temperature loading model 106, the loading line 124may be a curved path, such as an upwardly or downwardly sloping curve,or a series of linear paths having different slopes.

If the point (a, b) is disposed on (e.g., within tolerance of) theloading line 124 at decision block 122, then a step 128 computes a newRSSLV temperature reference value 90 by using the function F (L, V, T,Z). The function F (L, V, T, Z) is a transfer function of temperaturereference based on load (L), vibration (V), rate-controlled reheatingsteam temperature (T), and valve position (Z). The function F (L, V, T,Z) may be derived based on the RSSLV temperature reference value 90obtained by matching the current loading (L) of the loading line 124 ofFIG. 6 to the ordinate (y-axis) of the temperature loading model 106 ofFIG. 6, with constraints based on turbine hood side-to-side temperaturedifference to limit vibration (V), rate-controlled reheating steamtemperature (T) to maintain component safety, with coordination betweenall temperature controllers depending on valves CIV position/testing(Z).

In another embodiment, the RSLLV temperature reference value 90 may beobtained by matching the current loading of the loading line 124 to theordinate (y-axis) of the turbine loading model 106 and using the RSSLVtemperature reference value 90 found on the ordinate. In other words,given the current loading percentage as a, the process may determine b(i.e., the RSSLV temperature reference value 90) such that the point (a,b) falls on the loading line 124. Once a RSSLV temperature referencevalue 90 has been determined at step 128, then the setpoint 84 of thePID controllers 80 may be updated to reflect the newly determined RSSLVtemperature reference value 90 as described above with respect to FIG.3.

Continuing with decision 130 of the loading control logic, if thecurrent turbine load percentage is equal to or greater than load point Xof the temperature loading model 106 of FIG. 6, then at step 132 theRSSLV temperature reference value 90 is set to the maximum ratedreference temperature (i.e., 100% rated reheat temperature).Accordingly, the X point on the loading line 124 of the temperatureloading model 106 corresponds to a loading point at which thetemperature of the second stage reheater 42 steam is 100% of the ratedreheat temperature. Traditionally, X has been set to approximately 65%of loading. However, by using the disclosed closed-loop controlembodiments, the point X need not be at 65% of loading anymore. Indeed,the point X may now be chosen and updated, for example, by using a GUI,depending on factors such as turn-downs (e.g., anticipated shutdowns)and outage needs.

Returning to the parasitic loss control block 112, the control block isused to control situations in which the turbine 16 may be operatingclose to or over 100% rated reheat temperature and high loading, such asa portion 133 of the loading line 124 of FIG. 6. In this situation, theparasitic loss control block 112 logic may determine at decision block134 if the turbine loading percentage is greater than load point Z ofthe temperature loading model 106. If the turbine loading percentageexceeds load point Z, then the RSHLV 76 is opened at step 136. The RSHLV76 helps prevent a drop of pressure at high temperatures and turbineloadings by opening a second steam path in parallel with the first steampath created by using the RSLLV 74. If the parasitic loss control block112 determines at decision 138 that the turbine load percentage has comedown to less than the point Y of the temperature loading model 106, thenthe RSHLV 76 is closed at step 140. Thus, the parasitic loss controlblock 112 may be used to help alleviate or to prevent the parasitic lossof pressure at higher temperatures and turbine loadings.

Returning to the unloading control block 114, the control block is usedto control the second steam reheater 42 during unloading operations.Unloading occurs when less main steam is directed into the turbinesystem. The unloading control logic at decision 142 determines if thecurrent turbine load % is less than the loading point V of thetemperature loading model 106 of FIG. 6. If the current turbine load %is not less than V % load, then the RSLLV temperature reference 90 isheld at step 144 so that the unloading occurs along a horizontal line.For example, the RSLLV temperature reference value 90 may be held duringunloading, resulting in a horizontal unloading line (e.g., unloadinglines 146 and 148 of temperature loading model 106 of FIG. 6), todecouple MSR operations from normal load swings and also to preventunwanted thermal effects. Accordingly, a certain amount of unloading andloading operations, such as those depicted in unloading line 148 andloading line 150, may be tolerated while holding the same RSSLVtemperature reference value 90.

Continuing with decision 142 of the unloading control logic, if theunloading operation continues and reaches a point of less than V % loadof the temperature loading model 106 of FIG. 6, then the control logicmoves to decision block 152. Decision block 152 determines if thecurrent loading point (a, b) arrived by using the current turbine load %as a and the current sensed temperature as b is considered to be on(e.g., within a tolerance of) the unloading line 154 of FIG. 6. If thecurrent loading point (a, b) is not deemed to be on the unloading line154, then the RSLLV temperature reference value 90 may be held at step156 and the decision step 152 may be repeated until the point (a, b)returns to the unloading line 154. If decision 152 determines that theunloading is occurring on the unloading line 154 of FIG. 6, then thecontrol logic moves to step 158. In one embodiment, step 158 determinesthe RSSLV temperature reference value 90 by using the function F (L, V,T, Z) as described in more detail with respect to step 128 above.

In another embodiment, step 158 of the unloading control logicdetermines the RSLLV temperature reference value 90 by matching thecurrent loading of the unloading line 154 to the ordinate (y-axis) ofthe turbine loading model 106 and using the RSSLV temperature referencevalue 90 found on the ordinate. In other words, given the currentloading percentage as a, the step 158 may determine b (i.e., the RSSLVtemperature reference value 90) such that the point (a, b) falls on theunloading line 154.

The unloading control logic then moves to decision block 160. Decisionblock 160 determines if the unloading operation has reached a load pointU of the temperature loading model 106 of FIG. 6. If the unloading hasreached a load point U or has reached a load point less than U, then theMSR second stage reheater 42 is turned off at step 162 by closing theRSSLV 74.

Returning to the trip control block 116, the control block may be usedto control shutdown operations when a trip has occurred, for example,during an emergency condition. The tripping operation controlled by thetrip control block 116 includes two main steps. The RSHLV 76 is closedat step 140 and the RSLLV 74 is closed at step 162. Steps 140 and 162effectively turns off the MSR second stage reheater 42 by closing thetwo valves that may be used to feed steam into the reheater. The tripunloading line 164 of FIG. 6 graphically shows an example of how theRSSLV temperature reference value 90 may fall during a trip operationthat may occur at a load point of X. It is to be understood that a tripoperation may occur at other values of the abscissa, i.e., load axis,shown in the temperature loading model 106 of FIG. 6.

Returning to the auto shutdown control block 118, the control block maybe used when a controlled shutdown of the MSR second stage reheater 42is to be performed. Instead of immediately turning off the MSR, a loadpoint W, such as the point W depicted in FIG. 6, may be chosen as anintermediate shutdown point. The intermediate shutdown point may be usedto reduce temperature differences (and thermal stresses) while shuttingdown the MSR second stage reheater 42. The auto shutdown control block118 is, therefore, capable of minimizing unwanted thermal effects byfirst calculating the slope of a line that would reach point W from thecurrent load point at step 166. Two example shutdown lines 168 and 170have been calculated and are shown in FIG. 6. Line 168 starts at a loadpoint of X and line 170 starts at a load point close to W. Both lines168 and 170 end at load point W. In one embodiment, step 166 calculatesthe RSSLV temperature reference value 90 as the loading moves down ashutdown line such as lines 168 and 170 by using the function F (L, T,V, Z) as described in more detail above with respect to step 128.

In another embodiment, step 166 of the auto shutdown logic calculatesthe RSSLV temperature reference value 90 by matching the current loadingalong the appropriate shutdown line (e.g., 168 and 170) to the ordinate(y-axis) of the turbine loading model 106 and using the RSSLVtemperature reference value 90 found on the ordinate. In other words,given the current loading percentage as a, the step 166 may determine b(i.e., the RSSLV temperature reference value 90) such that the point (a,b) falls on the appropriate shutdown line. The RSSLV temperaturereference value 90 is continuously calculated until the load point W isreached. Decision 172 may then determine if the current turbine loadpercentage is less than or equal to the W % load. If so, the RSLLV 74may be closed at step 162 and the MRS second stage reheater 42 may beturned off.

It is to be understood that like points U and X, points V, W, X, Y, andZ may be easily changed through the use of a GUI. Further, the disclosedembodiments not only allow for flexibility in choosing the variousloading points, but also allow for the easy creation of a plurality oftemperature loading models, such as the example loading model depictedin FIG. 6. Indeed, the methods and apparatuses provided by the disclosedembodiments may be used to drastically increase the performance,efficiency, and safety of a power plant.

Technical effects of the invention include direct, closed-loop controlof the desired reheat temperature at each MSR 32 outlet, reduced steamturbine temperature transients that may reduce clearance rubs, reducedside-to-side temperature differences that may reduce vibration,increased coordination between the controllers controlling a pluralityof MSRs 32 in a power plant, and the use of a controlled rate oftemperature increases and decreases that respect the steam turbinematerial limitations. Closed-loop control embodiments are employed thatallow for the flexible implementation of various temperature loadingmodels as well as for the use of temperature reference functions. Thetemperature reference functions can incorporate data from a plurality ofsensors, including pressure sensors, flow sensors, temperature sensors,valve position sensors, clearance sensors, speed sensors, vibrationsensors, or a combination thereof, in order to calculate a safe andefficient temperature reference. The temperature reference may then beused as feedback to control the reheating of steam. Electronic signalsrather than pneumatic controls are used, resulting in faster and morereliable control embodiments.

This written description uses examples to disclose the invention,including the best mode, and also to enable any person skilled in theart to practice the invention, including making and using any devices orsystems and performing any incorporated methods. The patentable scope ofthe invention is defined by the claims, and may include other examplesthat occur to those skilled in the art. Such other examples are intendedto be within the scope of the claims if they have structural elementsthat do not differ from the literal language of the claims, or if theyinclude equivalent structural elements with insubstantial differencesfrom the literal languages of the claims.

1. A method for operating a turbine system, comprising: supplying firstmain steam from a main steam source to a high pressure steam turbine toenable the high pressure steam turbine to rotate a shaft in the turbinesystem using the main steam; supplying exhaust steam from the highpressure steam turbine to a moisture separator reheater to enable themoisture separator reheater to produce thermally enhanced steam byreheating the exhaust steam using second main steam received from themain steam source; supplying the thermally enhanced steam from themoisture separator reheater to a low pressure steam turbine to enablethe low pressure steam turbine to rotate the shaft using the thermallyenhanced steam; determining, using one or more sensors, a measuredtemperature of the thermally enhanced steam output from the moistureseparator reheater; and controlling supply of the second main steam fromthe main steam source to the moisture separator reheater based at leastin part on the measured temperature to facilitate substantially linearlyramping temperature of the thermally enhanced steam.
 2. The method ofclaim 1, wherein controlling the supply of the second main steamcomprises adjusting target temperature of the thermally enhanced steamproportionally with changes in loading percentage of the turbine systemwhile performing a loading operation, an unloading operation, or both.3. The method of claim 1, wherein controlling the supply of the secondmain steam comprises: determining, using a control system, a targetvalve position of a valve fluidly coupled between the main steam sourceand the moisture separator reheater; and instructing, using the controlsystem, the valve to adjust valve position based at least in part on thetarget valve position to facilitate controlling the temperature ofthermally enhanced steam.
 4. The method of claim 3, wherein determiningthe target valve position comprises: determining, using the controlsystem, a first temperature reference setpoint that ramps substantiallylinearly based on load percentage of the turbine system; and determiningthe target valve position based at least in part on the firsttemperature reference setpoint and the measured temperature of thethermally enhanced steam.
 5. The method of claim 4, wherein determiningthe target valve position comprises: determining a maximum ramp rate ofthe temperature of thermally enhanced steam; determining the targetvalve position based at least in part on difference between the firsttemperature reference setpoint and the measured temperature when thefirst temperature reference setpoint does not differs from the measuredtemperature by more than the maximum ramp rate; and when the firsttemperature reference setpoint differs from the measured temperature bymore than the maximum ramp rate: determining a second temperaturereference setpoint based at least in part on the maximum ramp rate; anddetermining the target valve position based at least in part ondifference between the measured temperature and the second temperaturereference setpoint.
 6. The method of claim 1, comprising determining,using a control system, a loading model that describes relationshipbetween turbine load percentage and target temperature of the thermallyenhanced steam; wherein controlling the supply of the second main steamcomprises: determining, using the control system, a temperaturereference setpoint based at least in part on the loading model and acurrent load percentage of the turbine system; and instructing, usingthe control system, a valve fluidly coupled between the main steamsource and the moisture separator reheater to control the supply of thesecond main steam to the moisture separator reheator based at least inpart on the temperature reference setpoint.
 7. The method of claim 6,wherein determining the loading model comprises: determining a loadingline to be used to determine the temperature reference setpoint whilethe turbine system performs a loading operation, wherein the temperaturereference increases proportionally with increases in the load percentageof the turbine system; and determining an unloading line to be used todetermine the temperature reference setpoint while the turbine systemperforms an unloading operation, wherein the temperature referencedecreases proportionally with decreases in the load percentage of theturbine system.
 8. The method of claim 6, wherein determining thetemperature reference comprises: determining the temperature referencesetpoint as a first temperature corresponding with the current loadpercentage in the loading model when the first temperature differs fromthe measured temperature by less than a maximum ramp rate; anddetermining the temperature reference setpoint as a second temperatureby adjusting the measured temperature based at least in part on themaximum ramp rate when the first temperature differs from the measuredtemperature by more than the maximum ramp rate.
 9. A tangible,non-transitory, computer-readable medium that stores instructionsexecutable by one or more processors of a control system to controloperation of a turbine system, wherein the instructions compriseinstructions to: instruct, using the one or more processors, a firstvalve fluidly coupled between a high pressure steam turbine and a mainsteam source to enable supply of main steam from the main steam sourceto the high pressure steam turbine to facilitate rotating a shaft in theturbine system using the main steam; instruct, using the one or moreprocessors, a second valve fluidly coupled between the moistureseparator reheater and a low pressure turbine to enable supply ofthermally enhanced steam from the moisture separator reheater to the lowpressure steam turbine to facilitate rotating the shaft using thethermally enhanced steam; determine, using the one or more processors,measured temperature of the thermally enhanced steam based at least inpart on sensor data received from one or more sensors; and instruct,using the one or more processors, a third valve fluidly coupled betweenthe main steam source and the moisture separator reheater to adjust afirst valve position to facilitate substantially linearly rampingtemperature of the thermally enhanced steam by controlling amount of themain steam available to heat the exhaust steam in the moisture separatorreheater.
 10. The computer-readable medium of claim 9, comprisinginstructions to instruct, using the one or more processors, a fourthvalve fluidly between the high pressure turbine and the moistureseparator reheater to adjust a second valve position to facilitatesubstantially linearly ramping the temperature of the thermally enhancedsteam by controlling amount of the exhaust steam available to heat theexhaust steam in the moisture separator reheater.
 11. Thecomputer-readable medium of claim 9, wherein the instructions toinstruct the third valve to adjust the first valve position compriseinstructions to: determine a first target temperature of the thermallyenhanced steam that changes proportionally with changes in loadingpercentage of the turbine system during loading operations, unloadingoperations, or both; and instruct the third valve to adjust the firstvalve position from a current valve position to a target valve positionbased at least in part on the first target temperature and the measuredtemperature of the thermally enhanced steam.
 12. The computer-readablemedium of claim 11, wherein the instructions to instruct the third valveto adjust the first valve position comprise instructions to: determine aproportional term based at least in part on a proportional gain and adifference between the measured temperature and the first targettemperature of the thermally enhanced steam; determine an integral termbased at least in in part on an integral gain and an integral of thedifference between the measured temperature and the first targettemperature of the thermally enhanced steam; determine a derivative termbased at least in part on a derivative gain and a derivative of thedifference between the measured temperature and the first targettemperature of the thermally enhanced steam; and instruct the thirdvalve to adjust the first valve position based at least in part on a sumof the proportional term, the integral term, and the derivative term.13. The computer-readable medium of claim 11, wherein the instructionsto instruct the third valve to adjust the first valve position compriseinstructions to: instruct the third valve to adjust the first valveposition based at least in part on a first difference between themeasured temperature and the first target temperature of the thermallyenhanced steam when the first target temperature does not differ fromthe measured temperature by more than a temperature threshold; and whenthe first target temperature differs from the measured temperature bymore than the temperature threshold: determine a second targettemperature of the thermally enhanced steam by adding the temperaturethreshold to the measured temperature when the first target temperatureis greater than the measured temperature; determine the second targettemperature by subtracting the temperature threshold from the measuredtemperature when the first target temperature is less than the measuredtemperature; and instruct the third valve to adjust the first valveposition based at least in part on a second difference between themeasured temperature and the second target temperature of the thermallyenhanced steam.
 14. The computer-readable medium of claim 9, wherein theinstructions to instruct the third valve to adjust the first valveposition comprise instructions to: adjust the first valve positiontoward a fully open position when a target temperature of the thermallyenhanced steam is greater than the measured temperature of the thermallyenhanced steam; and adjust the first valve position toward a fullyclosed position when the target temperature is less than the measuredtemperature of the thermally enhanced steam.
 15. The computer-readablemedium of claim 9, comprising instructions to determine, using the oneor more processors, a loading model that describes relationships betweenloading percentage of the turbine system and target temperature of thethermally enhanced steam, wherein the loading model comprises: a loadingline that describes a first relationship used when the turbine systemperforms a loading operation to linearly increase the target temperatureas the loading percentage of the turbine system increases; and anunloading line that describes a second relationship used when theturbine system performs an unloading operation to linearly decrease thetarget temperature as the loading percentage of the turbine systemdecreases.
 16. A turbine system comprising: a moisture separatorreheater comprising a first heat exchanger, wherein: the first heatexchanger is configured to receive first main steam from a main steamsource; the moisture separator reheater is configured to receive exhauststeam output from a first steam turbine after second main steam receivedfrom the main steam source is used to actuate a shaft in the turbinesystem; and the moisture separator reheater is configured to outputthermally enhance steam to be used by a second steam turbine to actuatethe shaft by flowing the exhaust gas around the first heat exchanger; atemperature sensor coupled to an output of the moisture separatorreheater, wherein the temperature sensor is configured to determine ameasured temperature of the thermally enhanced steam; and a first valvefluidly coupled to the first heat exchanger, wherein the first valve isconfigured to adjust supply of the first main steam to the first heatexchanger based at least in part on the measured temperature tofacilitate substantially linearly ramping temperature of the thermallyenhanced steam.
 17. The turbine system of claim 16, comprising a secondvalve fluidly coupled between the first steam turbine and a second heatexchanger in the moisture separator reheater, wherein: the second heatexchanger is configured to receive extraction steam from the firstturbine; the moisture separator reheater is configured to flow theexhaust gas around the second heat exchanger to facilitate producing thethermally enhanced steam; and the second valve is configured to adjustsupply of the extraction steam to the second heat exchanger based atleast in part on the measured temperature to facilitate substantiallylinearly ramping the temperature of the thermally enhanced steam. 18.The turbine system of claim 16, comprising a controller communicativelycoupled to the temperature sensor and the first valve, wherein thecontroller is configured to: determine the measured temperature of thethermally enhanced steam based at least in part on sensor data receivedfrom the temperature sensor; determine a first target temperature of thethermally enhanced steam based at least in part on a loading percentageof the turbine system, wherein the first target temperature changesproportionally with changes in the loading percentage while the turbinesystem is performing a loading operation or an unloading operation; andinstruct the first valve to adjust valve position based at least in parton a current valve position and difference between the first targettemperature and the measured temperature of the thermally enhanced steamwhen the first target temperature and the measured temperature differ byless than a rate change threshold.
 19. The turbine system of claim 18,wherein, when the first target temperature and the measured temperaturediffer by more than the rate change threshold, the controller isconfigured to: determine a second target temperature threshold byadjusting the measured temperature of the thermally enhanced steam basedat least in part on the rate change threshold; and instruct the valve toadjust the valve position based at least in part on the current valveposition and difference between the second target temperature and themeasured temperature of the thermally enhanced steam.
 20. The turbinesystem of claim 16, wherein: the first steam turbine comprises a highpressure steam turbine configured to facilitate rotating the shaft byflowing the main steam over a first one or more turbine blades coupledto the shaft; and the second steam turbine comprises a low pressuresteam turbine configured to facilitate rotating the shaft by flowing thethermally enhanced steam over a second one or more turbine bladescoupled to the shaft.